In this day and age, when many people are free to marry whomever they please, it might come as a surprise that royals aren't always encouraged or even allowed to marry their chosen mates due to color or creed.
In Britain, for example, successors to the throne must be members of the Church of England. Furthermore, if they marry a Catholic, it's hasta la vista to their chance at ruling. A princess can be next in line for ascension, but if she chooses to exchange "I do's" with a Catholic, she'll lose her eligibility to the throne. There has been some debate about loosening this restriction, but as of now it remains, for the most part, in effect.